1. Field of the Invention
This invention relates to the field of active control of axial fan rotor-stator interaction noise.
2. Prior Art
Communities located adjacent to commercial airports are often exposed to excessive and annoying noise generated from landing, takeoff and flyover maneuvers. FIG. 1 is a schematic cross-section of the most common propulsion system used in commercial high-bypass-ratio turbofan engines (see Turbomachinery Noise, Groeneweg, J. F., Sofrin, T. G. and Rice, E. J., NASA Reference Publication 1258, Vol. 1, WDRC Technical Report 90-3052, August 1991). The various internal noise sources are identified as well as locations of passive sound absorbing treatment. FIG. 2 displays predicted flyover maximum perceived noise levels generated from separate engine components of a typical commercial turbofan engine (taken from Energy Efficient Engine Propulsion System-Aircraft Integration Evaluation, Owens, R. E., NASA CR-159488, 1979). Observe that for this engine, the maximum perceived noise levels are dominated by fan inlet and exhaust sources. FIG. 3 displays sound power spectra generated from typical turbomachinery operating at subsonic and supersonic tip speeds. At subsonic tip speeds, large tones are observed at harmonics of the rotor blade-passage frequencies (BPF) in contrast to the spectra at supersonic tip speeds where very large number of tones are generated from rotating shock waves and associated nonlinearities at frequencies both above and below the engine blade passing frequency (BPF).
Despite many years of intensive research, jet engine noise remains as one of the major pollution problems facing communities located near civilian airports. This is not surprising because the suppression of jet engine noise is inherently complex, involving the interaction between different physical phenomena such as (1) complicated radial and spinning modes convecting in three-dimensional flows containing transverse velocity and thermal gradients which refract the sound, (2) subsonic and supersonic accelerating mean flows, (3) combustion noise, (4) acoustic wave propagation and resonance and (5) natural or forced hydrodynamic and acoustic instabilities. As a consequence of the complexity of these mechanisms and their (nonlinear) interactions, very few "practical" guidelines have evolved to allow the engine designer to predict, let alone control, jet noise inlet and exhaust noise in a given design.
The need to improve aircraft performance and efficiency while decreasing community noise taxes the acoustic suppression capability of the sound absorbing treatment that line the ducts of turbofan engines. New ultra-high-bypass engines with shorter inlet and exhaust ducts have less room for acoustic treatment. Thus, more effective treatment is required than is currently available. Much of the treatment used currently has a very limited frequency range over which it is effective, that is, it is only effective for an single tone. If the treatment could be effective for several or all tones at the same time, than all of the treatment area would be available for each tone.
Active sound attenuation is a relatively old concept that has received considerable attention in recent years, primarily because the increasing availability of fast programmable signal processing hardware has made these systems viable for audio frequency applications. Although active noise control technology has been demonstrated to be very successful in many industrial noise control applications, it has been not been used in applications that are sensitive to the severe weight, size, ruggedness, reliability and energy constraints required in the control of excessive commercial jet engine noise.